Rising water from Hurricane Isaac in Lafitte has set off the sensors on Jefferson Parish's high-water vehicles, halting efforts to help remaining residents evacuate. Sheriff Newell Normand said Wednesday evening marked the first time since the Sheriff's Office bought the trucks after Hurricane Katrina that a flood grew too dire for them to proceed.

Mark Waller/The Times-PicayuneNational Guard vehicles arrive in Lafitte Wednesday afternoon as water rapid rose over roads. The guardsmen were going to help with last-minute sandbagging against the rising tide of Hurricane Isaac. Hours later, Jefferson Parish officials announced they could no longer use their high-water vehicles to extract residents who attempted to ride out the storm in coastal communities.

"This is the first time this has happened of all the times we have used these trucks since Katrina," including in flood-prone Lafitte, Normand said. The alarms go off on the trucks, he said, when water piles about chest-high.

Parish officials earlier on Wednesday warned residents that time was running short to escape surging currents stoked by Isaac in the waterways around the coastal communities of Lafitte, Barataria and Crown Point.

Many spots already were impassable in personal vehicles, so residents wanting out had to rely on rides from the high-water trucks.

Normand said the warnings prompted 25 people to seek such assistance. He said the Sheriff's Office will send those evacuees to state-run shelters across Louisiana.

He said officials then called off more rescues by truck because stranding one of the tall vehicles would cause an additional obstruction and problem.

Boats are also difficult to use in this situation, he said. The landscape around Lafitte includes dips and ridges, mingling flooded spots with dry spots and preventing boats from traveling far before running aground.

The approach by boat, he said, would be along one of the waterways. But right now, he said, "The current is so strong, it's dangerous."